Gelated colloid crystal precursor and gelated colloid crystal, and method and apparatus for preparing gelated colloid crystal

ABSTRACT

Gelled colloidal crystals obtained by ultraviolet irradiation gelation means proposed so far in the art are inadequate in term of homogeneous gelation as far as their deep portions, and gelled colloidal crystals obtained by use of gelation means relying upon a light source in the visible light range in place of that ultraviolet irradiation has several problems such as difficulty with selection of polymerization initiators and generation of gas bubbles. The object of the invention is to provide a colloidal crystal gelled homogeneously to within, from which those problems are eliminated. To this end, a colloidal solution using an aqueous liquid as a disperse medium with a monomer or macromer added thereto and camphorquinone, riboflavin or their derivative contained therein as a polymerization initiator is irradiated with light having a wavelength component in the range of at least 400 nm to 500 nm for the purpose of gelation, thereby providing a solution to the aforesaid problems.

ART FIELD

The present invention relates generally to a colloidal crystal to begelled, which is a precursor for a gelled colloidal crystal functioningas a photonic crystal, a gelled colloidal crystal that is obtained bygelling that colloidal crystal to be gelled by light irradiation, and aprocess and system for the preparation of that gelled colloidal crystal.More specifically, the invention is concerned with a gelled colloidalcrystal of improved optical quality, which is obtained by fixing orimmobilizing a colloidal crystal that remains flowing as it is, using apolymer gel.

BACKGROUND ART

In a so-called colloidal dispersion in which fine particles with auniform enough particle size are dispersed in a solvent, if conditionssuch as an increased concentration of fine particles and a decreasedconcentration of ions in the solvent are put in order, there is thenobtained a state where the fine particles are lined upthree-dimensionally and periodically into a crystal-like structure. Thecolloidal dispersion in such a state is referred to as a colloidalcrystal (for instance, see non-patent publication 1). That colloidalcrystal is a sort of so-called photonic crystal, and found to produce aunique optical phenomenon in response to light. For this reason,attention has recently been paid to possible applications of thecolloidal crystal to optical elements, as reported in some academicpapers (see non-patent publications 2 and 3).

However, the above colloidal crystal is, in a manner of speaking, a kindof fluid in a state with fine particles dispersed in a liquid; thearrangement of particles is susceptible of becoming out of order byenvironmental perturbations such as vibrations and temperature changes.In consideration of practical applications to optical elements, it hasbeen proposed to intentionally keep back the fluidity of the colloidalcrystal and immobilize the whole colloidal crystal with a polymer gel,as described in a lot of literatures (for instance, see patentpublications 1 to 4, and non-patent publications 4 and 5). According tothose proposals of how to immobilize colloidal crystals with polymergels, a polymerizable monomer (or macromer), a crosslinking agent and aphoto-polymerization initiator are added into a disperse medium. Then,the dispersion is irradiated with ultraviolet light to excite thephoto-polymerization initiator, thereby setting off polymerization forgelation.

Non-Patent Publication 1

-   -   “Colloidal Chemistry I” edited by the Chemical Society of Japan,        Tokyo Kagaku Dozin Co., Ltd., pp. 119-123 (“Colloidal Crystal”        at Chapter 7, Section 7.2)

Non-Patent Publication 2

-   -   “Photonic Crystal” translated by Fujii and Inoue, Corona Co.,        Ltd., published on Oct. 23, 2000

Non-Patent Publication 3

-   -   “Dictionary of Physics and Chemistry, Fifth Edition” edited by        Iwanami Shoten Co., Ltd., Chapter “Photonic Crystal”, Fourth        Issue, published on Apr. 25, 2000

Non-Patent Publication 4

-   -   Asher et al. J. Am. Chem. Soc. Vol. 116, 1994, pp. 4997-4998

Non-Patent Publication 5

-   -   Jethmalani and Ford, Chem. Mater. Vol. 8, 1996, pp. 2138-2146

Patent Publication 1

-   -   U.S. Pat. No. 5,281,370

Patent Publication 2

-   -   U.S. Pat. No. 6,187,599

Patent Publication 3

-   -   U.S. Pat. No. 5,898,004

Patent Publication 4

-   -   EP0482394A2

We have also made some attempts on the above gelation using ultravioletirradiation, wherein, on the basis of the prior art proposed so far,colloidal crystals are gelled by ultraviolet light irradiation forimmobilization, thereby preparing colloidal crystals of good singlecrystallinity. However, we have found that this method is still far awayfrom satisfactory gelling means because of having a lot of problems inthe following respects.

The first problem arises by reason of the absorption and scattering ofultraviolet light by material, as set out at the following (a) and (b).

(a) Generally in the ultraviolet range, the absorption and scattering oflight by material grow whereas the power of transmission of lightbecomes weak; especially as colloidal crystals gain thickness, light isunlikely to reach deep in them, offering problems in conjunction withhomogeneous gelation. If optical elements such as lenses, filters andlight diffusers used with an optical system for light irradiation aremade of plastics, it is then industrially favorable due to costreductions. In many cases, however, such elements can never be used inthe ultraviolet range. Further, ultraviolet radiation is harmful to thehuman body. Thus, in consideration of safety at worksite, it would bemore desirable to do without ultraviolet radiation.

(b) As there is a larger difference in the index of refraction betweenparticles that form colloidal crystals and a disperse medium, it allowsthe scattering of light by the particles to become so intense thatvarious properties of the colloidal crystals behaving as photoniccrystals are more enhanced (for instance, see non-patent publication 2).Increasing that refractive index difference could be achieved by use ofa material having as large a refractive index difference as possible,and titanium dioxide could be promising as such a material. However,titanium dioxide is a typical photocatalyst that is excited byultraviolet radiation, and upon ultraviolet irradiation, there is anundesired decomposition of organic polymer components contained in thecolloidal crystals to be gelled, resulting in limited application oftitanium dioxide.

To add up to this, the second problem arises by reason of the inabilityof ultraviolet light to transmit through the structure of a colloidalcrystal, as set forth at (c).

(c) That is, with the use of colloidal crystals as a photo-functionelement in mind, colloidal crystals sensitively acting on light havingwavelengths at or near 800 nm (700 to 1,000 nm) emitted out of a widelyavailable titanium-sapphire laser are of industrially vital importanceand in great demand. Colloidal crystals acting on light havingwavelengths at or near 800 nm are typical when their longest Braggwavelength is found at or near 800 nm. Theoretical calculation ofphotonic crystals have already taught that colloidal crystals developunique optical properties with respect to light having a wavelength ator near the longest Bragg wavelength in the most noticeable way (seenon-patent publication 2). According to the most fundamental guidelinefor designing colloidal crystals acting on a titanium-sapphire laseroscillating a specific wavelength, therefore, it is desired to designthem in such a way as to have their longest Bragg wavelength at or near800 nm.

Here the longest Bragg wavelength is explained. In general crystals, tosay nothing of colloidal crystals, electromagnetic waves are subjectedto Bragg reflection at a lattice plane group having a specificcrystallographic azimuth. The electromagnetic wave subjected to Braggreflection at a lattice plane group having a specific azimuth becomeslongest when it is vertically incident on and regularly reflected atthat lattice plane group, i.e., when there is a 90° Bragg reflection,and the ensuring wavelength is proportional to a lattice plane spacing.Further, one crystal has different lattice plane groups with variouscrystallographic azimuths. For instance, the wavelength resulting from a90° Bragg reflection at a lattice plane group with a maximum latticeplane spacing, such as a (111) plane in the case of a face-centeredcubic lattice, and a (110) plane in the case of a body-centered cubiclattice, is the longest Bragg wavelength for that crystal.

Accordingly, we have investigated the spectral characteristics of such acolloidal crystal, and found out that its transmittance with respect tolight having a wavelength shorter than about a half of the longest Braggwavelength is very low, as illustrated in FIG. 1. It is here noted thatthe particle material used in FIG. 1 is polystyrene having a particlesize of 173 nm and a particle volume fraction concentration of about10%.

It follows that when the longest Bragg wavelength is 800 nm, there is avery low transmittance with respect to light having a wavelength ofabout 400 nm or less, i.e., light in the ultraviolet range. In otherwords, with colloidal crystals acting on light having a wavelength at ornear 800 nm emitted from a titanium-sapphire laser, there is a problemthat with a conventional ultraviolet induction type polymerization, itis difficult to spread irradiation light fully all over the sample,offering an obstacle to homogeneous yet efficient gelation.

SUMMARY OF THE INVENTION Subject Matter of the Invention

Gelation by ultraviolet irradiation, proposed so far in the art, hasproblems to be solved in many respects as already mentioned, and isstill far away from perfect technical means for obtaining opticallyimproved, high-quality gelled colloidal crystals. Especially inconsideration of reducing colloidal crystals down to a practical levelfrom now on, it would be required to ensure homogeneous gelation of notonly the surfaces but also the deep parts of the crystals, andcommercialization would never be successful without achieving thisobject. With the ultraviolet irradiation, much is still left to bedesired in many respects. Primary objects of the invention are toprovide solutions to the above problems, thereby achieving a colloidalcrystal on a practical level with a stable reproducibility, i.e., agelled colloidal crystal that is homogeneously gelled to within, andprovide a process and system for the preparation of that gelledcolloidal crystal.

With the gelation means by ultraviolet irradiation proposed so far inthe art, there are three such problems (a), (b) and (c) arising from theuse of wavelengths in the ultraviolet range. From this fact, it has nowbeen found that those problems can never be overcome without any methodthat does not rely on ultraviolet radiation; that is, they can beovercome by use of a polymerization initiator that is excited by visiblelight. Polymerization initiation by visible light could be much wider inapplications than that by ultraviolet light.

However, even with conversion from ultraviolet light irradiation tovisible light irradiation for the purpose of obtaining opticallyuniform, high-quality gelled colloidal crystals by gelation of colloidalcrystals using an aqueous solution as a disperse medium, it has now beenfound that gelation is hard to achieve with no selection of apolymerization initiator capable of reacting with visible light. So far,numerous polymerization initiators of the photo-excitation type,including the ultraviolet excitation type, have been proposed and usedin the art. However, experimentation has shown that all suchpolymerization initiators are not always effective for obtaininghomogeneously gelled crystals that are the object of the invention.

Therefore, some guidelines for selection of polymerization initiatorsare required; it has been found that the polymerization initiator usedmust satisfy three requirements given below. More specifically for theinvention wherein an aqueous liquid is used as a disperse medium, thephoto-polymerization initiator must have good enough water solubility,yet it must in strict sense be not insoluble. In other words, thepolymerization initiator in a molecular state must be dissolved in thedisperse medium composed of an aqueous liquid. This is because even inthe presence of slight insolubles, optical defects are introduced inpost-gelation colloidal crystals, rendering the advantage of theinvention meaningless. It has also been found that the polymerizationinitiator used must be of nonionic (or weakly ionic) nature. The reasonis that colloidal crystals are sensitive to the concentration of ions inthe solution, and so even slight changes or increases in the ionconcentration cause distortion or disintegration of crystals, againrendering the advantage of the invention meaningless. Another reason isthat the ionic initiator makes the concentration of ions in the solutionhigh, causing crystals to become non-homogeneous and, at worst, todisintegrate.

Further, when sensible tradeoffs are needed between increased crystalsize and more enhanced optical homogeneity, care should be taken toprevent formation of gas bubbles, because the presence of gas bubbles isa direct cause of optical defects. However, some polymerizationinitiators incur formation of gas bubbles, as can be seen from theexamples to be given later.

As described above, when such gelation as intended herein is achieved,not only is it required to convert irradiating light from ultravioletlight to visible light but there is also much difficulty in the changingand selection of the polymerization initiator. It has thus been foundthat it is very difficult to carry out gelation in an easy fashion.

Referring here to the gas bubble problem resulting from the aforesaidpolymerization initiator, it has been found that, for instance, the useof an initiator containing an azo group may incur generation of nitrogenupon light irradiation, which in turn causes precipitation of gasbubbles and inclusion of optical defects. However, the gas bubbleproblem occurring from the selection of the azo group is little or nonoticeable when colloidal crystals in an ordinary polycrystalline stateare prepared. This gas bubble problem has been unveiled for the firsttime as a result of our intensive study of seeking colloidal crystals ofhigh homogeneity by immobilization-by-gelation. In other words, thoseskilled in the art would have ever failed to recognize the gas bubbleproblem deriving from the initiator, because it was hidden by anon-homogeneity problem of the crystal sample itself, and there was noreference at all to the fact that it was caused by gasification.Referring further to this gas bubble problem, its early sign was alreadyobserved in our co-pending patent application No. 2000-217660 concerninga colloidal crystal preparation process. Since then, we were in for someproblems such as the gas bubble problem due to polymerization initiatorsand the ensuring optical defect problem, and studied what reason theyarose for. The result of that study is the invention that provides thelatest information and findings.

Means for Achieaving the Subject Matter

Based on the foregoing prior arts and a series of associated experimentsand studies, the present invention has for its object the provision of agelled colloidal crystal and its preparation process and system, whichare all free from various problems with the prior arts. As a consequenceof intensive and extensive studies, we have now found that if a specificpolymerization initiator capable of meeting all requirements regardingwater solubility, nonionic nature and no generation of gas bubbles andvisible light irradiation are selectively used for conditions capable ofobtaining gelled colloidal crystals free from the aforesaid problems,the desired gelled colloidal crystals can then be obtained. That is, wehave found that colloidal crystals can be successfully gelled with nodifficulty, if camphorquinone or riboflavin effective as thepolymerization initiator capable of meeting such requirements is used incombination with a specific light source.

Having accomplished on the basis of a series of findings and successes,the present invention is embodied as set out at (1) to (33) below.

(1) A colloidal crystal to be gelled by light irradiation, using anaqueous liquid as a disperse medium and at least comprising apolymerizable monomer or macromer, a crosslinking agent and aphoto-polymerization initiator, characterized in that camphorquinone,riboflavin, or their derivative is selectively used as saidphoto-polymerization initiator.

(2) A gelled colloidal crystal, which uses an aqueous liquid as adisperse medium, at least comprises a polymerizable monomer or macromer,a crosslinking agent and a photo-polymerization initiator, and is gelledby light irradiation, characterized in that camphorquinone, riboflavin,or their derivative is selectively used as said photo-polymerizationinitiator.

(3) The gelled colloidal crystal according to (2) above, wherein lighthaving a wavelength component in a range of at least 400 nm to 500 nm isused for said light irradiation.

(4) The gelled colloidal crystal according to (2) or (3) above, whichcontains titanium dioxide.

(5) A gelled colloidal crystal preparation process, characterized inthat a colloidal crystal to be gelled by light irradiation, using anaqueous liquid as a disperse medium and at least comprising apolymerizable monomer or macromer, a crosslinking agent and aphoto-polymerization initiator, wherein camphorquinone, riboflavin, ortheir derivative is selectively used as said photo-polymerizationinitiator, is irradiated with light to polymerize said monomer ormacromer for gelation of said colloidal crystal, thereby obtaining agelled colloidal crystal.

(6) The gelled colloidal crystal preparation process according to (5)above, characterized in that light having a wavelength component in arange of at least 400 nm to 500 nm is used for said light irradiation.

(7) The gelled colloidal crystal preparation process according to (5) or(6) above, characterized in that said colloidal crystal to be gelled hasa longest Bragg wavelength set at 700 to 1,000 nm.

(8) The gelled colloidal crystal preparation process according to (5) or(6) above, characterized in that said colloidal crystal to be gelledcontains titanium dioxide.

(9) The gelled colloidal crystal preparation process according to (7)above, characterized in that said colloidal crystal to be gelled furthercontains titanium dioxide.

(10) The gelled colloidal crystal preparation process according to (5)or (6) above, characterized in that said light irradiation source is ablue discharge lamp or a blue light-emitting diode.

(11) The gelled colloidal crystal preparation process according to (7)or (8) above, characterized in that said light irradiation source is ablue discharge lamp or a blue light-emitting diode.

(12) The gelled colloidal crystal preparation process according to (9)above, characterized in that said light irradiation source is a bluedischarge lamp or a blue light-emitting diode.

(13) The gelled colloidal crystal preparation process according to (5)or (7) above, characterized in that said light irradiation source is ablue laser having an emission wavelength in a range of 400 nm to 500 nmor a near infrared layer having an emission wavelength in a range of 800nm to 1,000 nm.

(14) The gelled colloidal crystal preparation process according to (8)or (9) above, characterized in that said light irradiation source is ablue laser having an emission wavelength in a range of 400 nm to 500 nmor a near infrared layer having an emission wavelength in a range of 800nm to 1,000 nm.

(15) The gelled colloidal crystal preparation process according to (10)above, characterized in that said blue discharge lamp is a blue neonlamp, a blue fluorescent lamp or a blue metal halide lamp.

(16) The gelled colloidal crystal preparation process according to (11)above, characterized in that said blue discharge lamp is a blue neonlamp, a blue fluorescent lamp or a blue metal halide lamp.

(17) The gelled colloidal crystal preparation process according to (12)above, characterized in that said blue discharge lamp is a blue neonlamp, a blue fluorescent lamp or a blue metal halide lamp.

(18) The gelled colloidal crystal preparation process according to (10)above, characterized in that said blue light-emitting diode is a GaNblue light-emitting diode.

(19) The gelled colloidal crystal preparation process according to (11)above, characterized in that said blue light-emitting diode is a GaNblue light-emitting diode.

(20) The gelled colloidal crystal preparation process according to (12)above, characterized in that said blue light-emitting diode is a GaNblue light-emitting diode.

(21) The gelled colloidal crystal preparation process according to (13)above, characterized in that said blue laser is an Nd:YAG laser thatemits light of 473 nm in wavelength or a blue oscillating GaN laser, anargon laser that oscillates light of 458 nm or 488 nm, or an He—Cd laserthat oscillates light of 442 nm.

(22) The gelled colloidal crystal preparation process according to (14)above, characterized in that said blue laser is an Nd:YAG laser thatemits light of 473 nm in wavelength or a blue oscillating GaN laser, anargon laser that oscillates light of 458 nm or 488 nm, or an He—Cd laserthat oscillates light of 442 nm.

(23) The gelled colloidal crystal preparation process according to (13)above, characterized in that said near infrared laser is an Nd:YAG laserthat oscillates light of 946 nm in wavelength, or a titanium sapphirelaser having an oscillation wavelength range of 800 nm to 1,000 nm.

(24) The gelled colloidal crystal preparation process according to (14)above, characterized in that said near infrared laser is an Nd:YAG laserthat oscillates light of 946 nm in wavelength, or a titanium sapphirelaser having an oscillation wavelength range of 800 nm to 1,000 nm.

(25) A gelled colloidal crystal preparation system for gelling acolloidal crystal to be gelled, which uses an aqueous liquid as adisperse medium and at least comprises a polymerizable monomer ormacromer, a crosslinking agent, and camphorquinone, riboflavin, or theirderivative as a photo-polymerization initiator, characterized in thatsaid preparation system further comprises blue light as a irradiationlight source.

(26) The gelled colloidal crystal preparation system according to (25)above, characterized in that said irradiation light source is a bluedischarge lamp or a blue light-emitting diode.

(27) The gelled colloidal crystal preparation system according to (26)above, characterized in that said blue discharge lamp is a blue neonlamp, a blue fluorescent lamp or a blue metal halide lamp.

(28) The gelled colloidal crystal preparation system according to (26)above, characterized in that said blue light-emitting diode is a GaNblue light-emitting diode.

(29) The gelled colloidal crystal preparation system according to (25)above, characterized in that said irradiation light source is a bluelaser having an emission wavelength in a range of 400 nm to 500 nm, or anear infrared laser having an emission wavelength in a range of 800 nmto 1,000 nm.

(30) The gelled colloidal crystal preparation system according to (29)above, characterized in that said blue laser is an Nd:YAG laser thatemits light of 473 nm in wavelength or a blue oscillating GaN laser, anargon laser that oscillates light of 458 nm or 488 nm, or an He—Cd laserthat oscillates light of 442 nm.

(31) The gelled colloidal crystal preparation system according to (29)above, characterized in that said near infrared laser is an Nd:YAG laserthat oscillates light of 946 nm in wavelength, or a titanium-sapphirelaser having an oscillation wavelength range of 800 nm to 1,000 nm.

(32) The gelled colloidal crystal preparation system according to (29)or (30) above, characterized by comprising scanning means capable ofscanning a laser light irradiation site in any spatial configuration.

(33) The gelled colloidal crystal preparation system according to (31)above, characterized by comprising scanning means capable of scanning alaser light irradiation site in any spatial configuration.

The foregoing embodiments of the invention are quite unique over theprior art, and have unique advantages as well. That is, conventionalmethods known as technical means for initiating polymerization by lightirradiation to gelate colloidal crystals rely exclusively on ultravioletlight irradiation, whereas the invention makes use of blue wavelengthsin the visible light range. The invention is also novel and superior inthat camphorquinone or riboflavin is selectively used as thepolymerization initiator.

As already known in the art, camphorquinone or riboflavin per se is apolymerization initiator by visible light. However, camphorquinone hasbeen used mainly in the field of dental materials, in which the use ofultraviolet radiation is inhibited exclusively by reason of influenceson the human body, and riboflavin has often been applied to thepreparation of acrylamide gels for electrophoresis. However, never untilnow is there any case or report of selectively using them as initiatorsfor gelation of colloidal crystals. Camphorquinone or riboflavin has alight absorption band at or near the light wavelength of 460 nm used asthe exciting wavelength for photo-polymerization initiation. However,the incorporation of this in colloidal crystals means that the resultinggelled colloidal crystals, too, have an absorption band at thatwavelength. Thus, it is beyond expectation to selectively usecamphorquinone or riboflavin that results in unavoidable incorporationof the absorption band at that wavelength.

By intentional adoption of camphorquinone or riboflavin, we havesucceeded in achieving optically quite strictly homogeneous gelationthrough irradiation with light in the visible light range with no use ofharmful ultraviolet radiation. We have also been able to provide asolution to the problems difficult to solve by use of otherpolymerization initiators.

Moreover, we have found out that the aforesaid absorption band problemwith the use of camphorquinone or riboflavin, too, can be practicallysolved by using them in a controlled or decreased amount. According tothe invention, camphorquinone or riboflavin has an absorption spectrumthat shows a large absorption in a blue range (400 nm to 500 nm) peakingat about 460 nm, and so allows polymerization to start upon irradiationwith blue visible light. In this connection, we have confirmed thatalthough camphorquinone or riboflavin is not that high in watersolubility, they have a water solubility good enough to work effectivelyas a photo-polymerization initiator and can provide optical defect-freeyet high-quality gelled colloidal crystals without generating any gasbubbles upon polymerization reaction. Besides, we have ascertained thatderivatives of camphorquinone or riboflavin, too, work as an effectivephoto-polymerization initiator as long as they maintain activity as aphoto-polymerization initiator. A series of such findings aboutcamphorquinone and riboflavin have underlain the present invention.

ADVANTAGES OF THE INVENTION

According to the invention, colloidal crystals blended with apolymerizable monomer or macromer and a specific componentpolymerization initiator is irradiated with blue light in the visiblerange, thereby succeeding in homogeneously gelling and immobilizing thecolloidal crystals having fluidity, and the resulting gelled colloidalcrystals are more improved in quality and larger in size than thoseobtained by ultraviolet irradiation. Therefore, the invention isbelieved to give a push and make a lot of contribution to theutilization and practical use of colloidal crystals in the form ofoptical elements, and so is of ever greater significance.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is illustrative of the dependency on wavelength of the lighttransmittance of a colloidal crystal that has a longest Bragg wavelengthof about 750 nm and is of good single crystallinity.

FIG. 2 is illustrative in schematic of the gelation system of theinvention for preparing homogeneously gelled colloidal crystals.

FIG. 3 is illustrative in schematic of the gelation system of theinvention for preparing homogeneously gelled colloidal crystals.

FIG. 4 is illustrative in schematic of the system for preparing geldedcolloidal crystals of given shape.

BEST MODE FOR CARRYING THE INVENTION

In the embodiments of the invention, especially in embodiments (3), (6),(10) to (24) and (26) to (31), some limitation is imposed on theirradiation light used for gelation or its source, because the light andits source recited therein are particularly preferable forcamphor-quinone, riboflavin or their derivative; this ensures larger andmore homogeneous gelation than do conventional gelation means relying onlight in the ultraviolet range. That is, the specific polymerizationinitiator cooperates with the specific light source to make it possibleto obtain gelled colloidal crystals that are larger in size and moreimproved in quality than ever before, or gelled colloidal crystals ofany desired shape. The advantages of the invention are far morefavorable than those obtained by gelation means relying upon light inthe ultraviolet range or other polymerization initiators. Especiallywhen camphorquinone or riboflavin is used as the polymerizationinitiator, it is preferable in view of energy efficiency that a bluelight source intensively containing a wavelength component at or near460 nm is used as the irradiation light source.

It has been found that blue discharge lamps (such as blue neon lamps,blue fluorescent lamps, and blue metal halide lamps), bluelight-emitting diodes, blue solid lasers and blue gas lasers aresuitable for such light sources. Although the blue neon lamp actuallyuses argon gas and mercury instead of neon gas, it usually has thecommon name of neon. Accordingly, it is favorable to use those lightsources for exciting light for polymerization initiation. Morespecifically, the blue light-emitting diode includes a GaNlight-emitting diode with a center wavelength of about 470 nm; and theblue solid laser includes an Nd:YAG laser emitting a 473-nm wavelengthlight that is a second harmonics having an oscillation wavelength of 946nm and a GaN laser of 410 nm. Exemplary blue gas lasers are an argonlaser of 458 nm or 488 nm and an He—Cd laser of 442 nm.

Especially the blue discharge lamp and blue light-emitting diode of theblue light sources, because of being divergent light sources, are wellsuited for large-area irradiation. Use of the blue discharge lamp or theblue light-emitting diode as the light source ensures that a large areais so irradiated with light of uniform intensity that the whole samplecontaining camphorquinone or riboflavin as the initiator can besimultaneously and homogeneously gelled to obtain gelled colloidalcrystals of large size.

On the other hand, the blue laser having an emission wavelength in therange of 400 nm to 500 nm, and the near infrared laser that is aseffective as the blue laser as a result of the so-called two-photonabsorption and has an emission wavelength in the range of 800 nm to1,000 nm twice as long as that of blue laser light is well suited forthe gelation of a specific narrow region. If, as recited in embodiment(32) or (33), a sample is irradiated and scanned by scanning means inany desired spatial configuration, it is then possible to gel the samplein any desired shape, thereby obtaining gelled colloidal crystals ofgiven shape.

An account is now given of why the near infrared laser having awavelength twice as long as that of blue laser light is as effective ongelation as the blue laser.

In ordinary photo-polymerization, a photo-polymerization initiator isexcited upon irradiation with light having a wavelength commensuratewith an excitation wavelength band for photo-polymerization, therebysetting off photo-polymerization; however, with irradiation with lighthaving other wavelengths, there is no polymerization because thephoto-polymerization initiator is never excited. Still, it has beenknown that upon irradiation with light of strong intensity just likelight having a wavelength twice as long as that of said excitationwavelength band, there is a unique phenomenon referred to as two-photonabsorption which in turn permits photons to be absorbed in thephoto-polymerization initiator for exciting it, so that just asirradiation with the light in the excitation wavelength band,polymerization can be started (for instance, see non-patent publications6, 7 and 8).

Non-Patent Publication 6

-   -   S. Kawata et al. Nature, Vol. 412, 16, Aug. 2001, pp. 697-698

Non-Patent Publication 7

-   -   Retrieved on Jan. 7, 2003 on the Internet:        http://biomicro.ikuta.mech.nagoya-u.ac.jp/researches/biochemical-j/2photon.html

Non-Patent Publication 8

-   -   Retrieved on Jan. 7, 2003 on the Internet:        http://www.nips.ac.jp/guide/res/cell.html

Such lasers as may be used for the excitation of camphorquinone orriboflavin include a pulse oscillation type Nd:YAG laser having anoscillation wavelength of 946 nm, and a pulse oscillation typetitanium-sapphire laser having an oscillation wavelength of 800 nm to1,000 nm.

In particular, embodiment (32) or (33) of the invention provides asystem that is well suited for obtaining gelled colloidal crystals notonly in a flat sheet form but also in a given form or pattern, and setup as recited therein.

This system ensures that upon gelation, a colloidal crystal is scannedand irradiated with light in a given form, so that it can be gelled inany desired form to obtain a homogeneously gelled, large-area colloidalcrystal. The system of the invention is favorable in view of preparationprocesses, because some complicated steps of cutting samples out of thegelled colloidal crystal can be dispensed with. To this end, the systemis designed such that only a specific site of the colloidal crystal tobe gelled, with camphorquinone or riboflavin contained therein as theinitiator, is spatially scanned and irradiated with a light beam fromthe blue laser light source or the near infrared laser having awavelength twice as long or its condensed light. It is thus possible togel only the site of the crystal irradiated with light, therebypreparing a gelled colloidal crystal of pre-designed shape.

One exemplary given form of gelled colloidal crystal is a round formthat is used for an optical filter as an example. As a large-area,sheet-like colloidal crystal to be gelled is circularly irradiated withlight at only the site to be gelled to make a plurality of gelledcolloidal crystals, it allows them to be prepared more efficiently thanthey are cut out of a wholly gelled single crystal. Thus, if gelation iscarried out in a form well fit for the purpose, it is then possible tomake preparation efficiency higher as compared with when crystal piecesare cut out of a large gelled colloidal crystal. A non-gelled portion ofthe crystal not irradiated with light, because of being in anon-solidified fluid state, may be washed away after the completion ofgelling operation, so that only the gelled portion can be removed assolid.

EMBODIMENTS OF THE INVENTION

The invention is now explained with reference to the examples anddrawings. It is understood, however, that given as an aid to a betterunderstanding of the invention, the examples are in no sense limited tothe invention.

Example 1

Submicron fine particles of uniform size (polystyrene or other polymerfine particles, various oxide particles inclusive of silica and titaniumdioxide particles, and any fine particles such as metal particles) weredispersed in water to prepare a colloidal dispersion. The colloidalsolution was adequately desalted by coexistence and contact with an ionexchange resin into a colloidal crystal state, thereby providing asolution 1.

Then, an aqueous solution of a polymerizable monomer that was a materialforming a gel network structure was provided as a solution 2 (forinstance, a 5M aqueous solution of acrylamide monomer), and acrosslinking agent aqueous solution as a solution 3 (for instance, a 0.1M aqueous solution of methylene-bis(acrylamide). Further, a saturatedaqueous solution of camphorquinone or riboflavin was provided as asolution 4.

These solutions 1, 2, 3, 4 and pure water were mixed together at amixing volume ratio of 1.5:0.8:0.3:0.15:0.25 to prepare a colloidalcrystal solution having a given volume fraction, through which an inertgas such as argon gas or nitrogen gas was passed to expel out dissolvedoxygen in the solution. The resulting solution was filled in a knockdowntransparent cell made of quartz glass or the like to make a colloidalcrystal sample to be gelled.

Then, the colloidal crystal to be gelled was irradiated with light froma visible light source containing a blue wavelength component, forinstance, a metal halide lamp or a halogen lamp.

In consideration of energy efficiency, a light source having a bluewavelength as a primary component is preferable for the light sourcegiving out irradiation light. Such a light source, for instance,includes a blue neon lamp, a blue fluorescent lamp, a blue metal halidelamp (put on the market as Color HID Lamp), a blue light-emitting diode,a blue solid laser, and a blue gas laser.

Among these, the blue light-emitting diode is commercially available asGaN diodes. For the blue solid laser, GaN lasers (410 nm) and Nd-YAGlasers (473 nm) are commercially available, and for the blue gas laser,argon lasers (458 nm or 488 nm) and He—Cd lasers (442 nm) are on themarket.

As the polymerization reaction was triggered by irradiation with lightcoming from the above light source, it caused polymerization of themonomer or macromer in the disperse medium, whereby the colloidalcrystal was gelled homogeneously as a whole. In this way, a gelledcolloidal crystal free from fluidity could be obtained.

The resulting gelled colloidal crystal was found to contain neitherinsoluble precipitates nor gas bubbles and their traces, both resultingfrom the polymerization initiator. That is, a gelled colloidal crystalthat was free from any optical defect derived from the initiator and soof high quality could be obtained. Camphorquinone or riboflavin having alight absorption band at or near 460 nm means that the post-gelationcolloidal crystal, too, has an absorption band at or near 460 nm.However, the amount of the initiator added was so small that influencesof that absorption could be as small as negligible in ordinaryapplications.

In Example 1, by proper selection of the size and concentration of theparticles used in solution 1, the lattice constant and the longest Braggwavelength of the finally obtained gelled colloidal crystal can becontrolled, and the properties of the colloidal crystal can be set insuch a way as to have any desired value inclusive of about 800 nm atwhich the longest Bragg wavelength is to be set.

Especially when titanium dioxide is used as the particles, ultravioletcomponents of 400 nm or less must be removed from irradiation light forthe purpose of holding back its photocatalyst reaction. Accordingly,when a light source containing such ultraviolet components is used asthe light source, it is preferable to fully cut out ultravioletradiation through an ultraviolet cut filter for irradiation purposes.

The gelation system of the invention is now explained with reference toFIGS. 2 to 4. Referring first to FIG. 2, the gelation system is embodiedas a light irradiator system 1 having a boxy vessel structure, theinside wall surface of which is made up of a material capable ofreflecting light. The vessel houses a blue light source 2 and, facingthis light source, the colloidal crystal sample 3 to be gelled, which isplaced in a transparent cell, is positioned via a light diffuser 6. Withactuation of a timer 5, the light source is put on to irradiate thecolloidal crystal sample to be gelled with light, whereuponcamphorquinone or riboflavin or their derivative added into and blendedwith the sample is excited by light and the polymerizable monomer ormacromer likewise incorporated in the sample starts polymerization, sothat the colloidal crystal is gradually gelled, and over time, thecolloidal crystal is immobilized while losing its fluidity. As the timerset in tune with the gelling time stops, it allows the light source tobe put off, bringing gelling operation to completion.

The explanation of the above preparation system and its operation modeis given for the purpose of illustration alone, not for the purpose oflimitation to the invention. Insofar as the objects of the invention areachievable, no particular limitation is imposed on the type of lightsource, what relation the light source and the sample are positioned in,elements and parts, what relations they are positioned in, etc., and soa good deal of possible design modes are included in the invention. Forinstance, a plurality of light sources could be positioned about thecolloidal crystal sample to be gelled for irradiation with light beamsfrom a plurality of positions or, alternatively, the sample and thelight source could be set such that they are relatively rotatable touniformly irradiate the sample with light.

Referring again to FIG. 2, the light diffuser 6 is interposed betweenthe light source 2 and the sample 3 so as to provide uniform irradiationof the sample with light through the light diffuser, ensuringhomogeneous gelation of the sample.

The invention seeks to achieve gelation by irradiation with light in thevisible light range, and the requirements to this end are that thepolymerization initiator be selected from camphorquinone, riboflavin ortheir derivative, and used in combination with a light source suitablefor that.

A variety of light sources could be used for that purpose. For instance,blue neon lamps, blue fluorescent lamps or blue metal halide lamps couldbe used as the blue discharge lamp in the system depicted in FIG. 2. Ifone selected from such lamps is used in combination with the colloidalcrystal sample to be gelled, which contains camphorquinone, riboflavinor their derivative as the initiator, it is then possible to obtain ahomogeneously gelled colloidal crystal of improved optical quality. Inone embodiment of the system of the invention for preparing the gelledcolloidal crystal wherein a plurality of GaN blue light-emitting diodesare used in place of such a discharge lamp, they are positioned on theinner surface or a portion of the inside cell or wall of the lightirradiation box.

The above light irradiator system set up as the gelation system has asingle unitary vessel structure. However, that system could also bedesigned in the form of a much handier type of light irradiator orgelation device, as shown in FIG. 3. More specifically, a blue lightirradiator device 8 such as an optical fiber irradiator device,comprising a blue discharge lamp or a GaN blue light-emitting diode as alight-emitting source 7 is used to guide light to a given position byway of an optical fiber 10.

Referring then to FIG. 4, there is depicted a system for the preparationof a gelled colloidal crystal of given shape. Briefly, this system isbuilt up of an XY plane translation mechanism 12 controlled by acomputer 14, and an optical fiber emitting terminal 11 attached thereto.Laser light emitted out of a blue (or near infrared) laser 9 is guidedto the emitting terminal 11 through an optical fiber 10, and theemitting terminal is freely movable in a plane vertical to the opticalaxis. Such an XY plane translation mechanism itself is well known in theart, and if it is combined with a commercially availableelectric-powered system, a computer controlled type system can then beeasily set up.

The emitting terminal 11 of the optical fiber is provided with acondensing lens that allows condensed light to be directed onto asample. On the way of an optical path running from the laser to theemitting terminal of the optical fiber, there is provided a computercontrolled type of optical shutter mechanism 13. The computer 14 isprogrammed such that given two-dimensional shape data, a correspondingportion of the sample is irradiated with light. By movement of theemitting terminal 11 of the optical fiber and opening/closing of theoptical shutter 13, the sample is irradiated with light in a givenconfiguration. If the colloidal crystal sample 3 to be gelled, whichcontains camphorquinone, riboflavin or their derivative as theinitiator, is used as the sample, it is then possible to obtain acolloidal crystal gel that is of given shape and improved opticalquality.

In view of the appropriateness of emitting wavelength and ease ofhandling, the most preferable type of blue laser 9 is an Nd:YAG laseremitting light having a wavelength of 473 nm that is a second harmonicof an oscillation wavelength of 946 nm. However, any of the aforesaidblue lasers could be used as the light source.

An exemplary near infrared laser is a pulse oscillating Nd:YAG laserhaving an oscillation wavelength of 946 nm, and a pulse oscillatingtitanium-sapphire laser having an oscillation wavelength of 800 nm to1,000 nm.

In a specific embodiment of light irradiation, the sample 3 to be gelledcould be moved in a plane vertical to the optical axis instead of lightscanning. In this case, a sample table 15 in FIG. 4 is designed as acomputer controlled type XY electric-powered stage, so that inassociation with the optical shutter 13, the sample can be irradiatedwith light in a given configuration.

POSSIBLE APPLICATIONS OF THE INVENTION TO THE INDUSTRY

The gelled colloidal crystal obtained according to the invention is moreimproved in quality and larger in size than ever before. The inventionis of quite vital significance, because it would encourage the use andpractical application of colloidal crystals as optical elements and makea great deal of contribution to the industry.

1. A colloidal crystal to be gelled by light irradiation, using anaqueous liquid as a disperse medium and at least comprising apolymerizable monomer or macromer, a crosslinking agent and aphoto-polymerization initiator, characterized in that camphorquinone,riboflavin, or their derivative is selectively used as saidphoto-polymerization initiator.
 2. A gelled colloidal crystal, whichuses an aqueous liquid as a disperse medium, at least comprises apolymerizable monomer or macromer, a crosslinking agent and aphoto-polymerization initiator, and is gelled by light irradiation,characterized in that camphorquinone, riboflavin, or their derivative isselectively used as said photo-polymerization initiator.
 3. The gelledcolloidal crystal according to (2) above, wherein light including awavelength component in a range of at least 400 nm to 500 nm is used forsaid light irradiation.
 4. The gelled colloidal crystal according toclaim 3 or 2 above, which contains titanium dioxide.
 5. A gelledcolloidal crystal preparation process, characterized in that a colloidalcrystal to be gelled by light irradiation, using an aqueous liquid as adisperse medium and at least comprising a polymerizable monomer ormacromer, a crosslinking agent and a photo-polymerization initiator,wherein camphorquinone, riboflavin, or their derivative is selectivelyused as said photo-polymerization initiator, is irradiated with light topolymerize said monomer or macromer for gelation of said colloidalcrystal, thereby obtaining a gelled colloidal crystal.
 6. The gelledcolloidal crystal preparation process according to claim 5,characterized in that light having a wavelength component in a range ofat least 400 nm to 500 nm is used for said light irradiation.
 7. Thegelled colloidal crystal preparation process according to claim 5 or 6,characterized in that said colloidal crystal to be gelled has a longestBragg wavelength set at 700 to 1,000 nm.
 8. The gelled colloidal crystalpreparation process according to claim 5 or 6, characterized in thatsaid colloidal crystal to be gelled contains titanium dioxide.
 9. Thegelled colloidal crystal preparation process according to claim 7,characterized in that said colloidal crystal to be gelled furthercontains titanium dioxide.
 10. The gelled colloidal crystal preparationprocess according to claim 5 or 6, characterized in that said lightirradiation source is a blue discharge lamp or a blue light-emittingdiode.
 11. The gelled colloidal crystal preparation process according toclaim 7 or 8, characterized in that said light irradiation source is ablue discharge lamp or a blue light-emitting diode.
 12. The gelledcolloidal crystal preparation process according to claim 9,characterized in that said light irradiation source is a blue dischargelamp or a blue light-emitting diode.
 13. The gelled colloidal crystalpreparation process according to claim 5 or 7, characterized in thatsaid light irradiation source is a blue laser having an emissionwavelength in a range of 400 nm to 500 nm or a near infrared layerhaving an emission wavelength in a range of 800 nm to 1,000 nm.
 14. Thegelled colloidal crystal preparation process according to claim 8 or 9,characterized in that said light irradiation source is a blue laserhaving an emission wavelength in a range of 400 nm to 500 nm or a nearinfrared layer having an emission wavelength in a range of 800 nm to1,000 nm.
 15. The gelled colloidal crystal preparation process accordingto claim 10, characterized in that said blue discharge lamp is a blueneon lamp, a blue fluorescent lamp or a blue metal halide lamp.
 16. Thegelled colloidal crystal preparation process according to claim 11,characterized in that said blue discharge lamp is a blue neon lamp, ablue fluorescent lamp or a blue metal halide lamp.
 17. The gelledcolloidal crystal preparation process according to claim 12,characterized in that said blue discharge lamp is a blue neon lamp, ablue fluorescent lamp or a blue metal halide lamp.
 18. The gelledcolloidal crystal preparation process according to claim 10,characterized in that said blue light-emitting diode is a GaN bluelight-emitting diode.
 19. The gelled colloidal crystal preparationprocess according to claim 11, characterized in that said bluelight-emitting diode is a GaN blue light-emitting diode.
 20. The gelledcolloidal crystal preparation process according to claim 12,characterized in that said blue light-emitting diode is a GaN bluelight-emitting diode.
 21. The gelled colloidal crystal preparationprocess according to claim 13, characterized in that said blue laser isan Nd:YAG laser that emits light of 473 nm in wavelength or a blueoscillating GaN laser, an argon laser that oscillates light of 458 nm or488 nm, or an He—Cd laser that oscillates light of 442 nm.
 22. Thegelled colloidal crystal preparation process according to claim 14,characterized in that said blue laser is an Nd:YAG laser that emitslight of 473 nm in wavelength or a blue oscillating GaN laser, an argonlaser that oscillates light of 458 nm or 488 nm, or an He—Cd laser thatoscillates light of 442 nm.
 23. The gelled colloidal crystal preparationprocess according to claim 13, characterized in that said near infraredlaser is an Nd:YAG laser that oscillates light of 946 nm in wavelength,or a titanium sapphire laser having an oscillation wavelength range of800 nm to 1,000 nm.
 24. The gelled colloidal crystal preparation processaccording to claim 14, characterized in that said near infrared laser isan Nd:YAG laser that oscillates light of 946 nm in wavelength, or atitanium sapphire laser having an oscillation wavelength range of 800 nmto 1,000 nm.
 25. A gelled colloidal crystal preparation system forgelling a colloidal crystal to be gelled, which uses an aqueous liquidas a disperse medium and at least comprises a polymerizable monomer ormacromer, a crosslinking agent, and camphorquinone, riboflavin, or theirderivative as a photo-polymerization initiator, characterized in thatsaid preparation system further comprises blue light as a irradiationlight source.
 26. The gelled colloidal crystal preparation systemaccording to claim 25, characterized in that said irradiation lightsource is a blue discharge lamp or a blue light-emitting diode.
 27. Thegelled colloidal crystal preparation system according to claim 26,characterized in that said blue discharge lamp is a blue neon lamp, ablue fluorescent lamp or a blue metal halide lamp.
 28. The gelledcolloidal crystal preparation system according to claim 26,characterized in that said blue light-emitting diode is a GaN bluelight-emitting diode.
 29. The gelled colloidal crystal preparationsystem according to claim 25, characterized in that said irradiationlight source is a blue laser having an emission wavelength in a range of400 nm to 500 nm, or a near infrared laser having an emission wavelengthin a range of 800 nm to 1,000 nm.
 30. The gelled colloidal crystalpreparation system according to claim 29, characterized in that saidblue laser is an Nd:YAG laser that emits light of 473 nm in wavelengthor a blue oscillating GaN laser, an argon laser that oscillates light of458 nm or 488 nm, or an He—Cd laser that oscillates light of 442 nm. 31.The gelled colloidal crystal preparation system according to claim 29,characterized in that said near infrared laser is an Nd:YAG laser thatoscillates light of 946 nm in wavelength, or a titanium-sapphire laserhaving an oscillation wavelength range of 800 nm to 1,000 nm.
 32. Thegelled colloidal crystal preparation system according to claim 29 or 30,characterized by comprising scanning means capable of scanning a laserlight irradiation site in any spatial configuration.
 33. The gelledcolloidal crystal preparation system according to claim 31,characterized by comprising scanning means capable of scanning a laserlight irradiation site in any spatial configuration.